Oxidation of alkenes or alkanes to ketones often produces crude ketone streams that contain variable amounts of aldehyde byproducts, among others. These aldehyde byproducts need to be removed from the crude ketone streams to meet the product specifications, of purity and/or odor, of the end product.
According to the prior art, the removal of aldehydes from ketones is commonly achieved in one of the following two ways: (1) by direct separation, using an appropriate physical unit operation, such as thermal distillation or physical adsorption; or, (2) by converting the aldehyde into a product which is more amenable to separation from the ketone than the parent aldehyde, followed by an appropriate separation of the reaction product. The latter approach typically involves the use of a chemical reaction, such as hydrogenation, aldol condensation or bisulfite adduction, and is especially preferred, if not required, when the physical properties of the aldehyde impurity and the desired ketone (such as boiling point or molecular size) are too similar to enable an economical direct separation (such as distillation).
Examples of the above methods for removing aldehydes from ketones that have been disclosed in the literature include: physical adsorption of the aldehyde on activated alumina or molecular sieves (Japanese Pat. No. J45-39083); hydrogenation of the aldehyde into the corresponding alcohol followed by distillation (Smidt J. and Krekeler H., 6th World Petroleum Congress, Section IV, Paper 40-PD9, Frankfurt, 1963); aldol condensation of the aldehyde into an aldol product followed by, or integrated with, distillation (U.S. Pat. No. 3,198,837; U.S. Pat. No. 3,392,200; Japanese Pat. No. J56-158725; U.S. Pat. No. 4,329,510; Japanese Patent No. J58-208246); and addition of the aldehyde to sodium bisulfite followed by removal of the bisulfite adduct (Hydrocarbon Processing, 204, November, 1969; Japanese Pat. No. J47-33323; Stewart T. D. and Donally L. H., Journal of the American Chemical Society, 54, 2333, 3555, 3559, 1932; Sorensen P. E. and Andersen V. S., Acta Chemica Scandinavia, 24, 1301, 1970).
In commercial application, ketone products often have rigorous odor specifications, which require the elimination of substantially all aldehydic impurities which are oderiferous themselves or are the precursors of readily formed oderiferous compounds. As a result, final, commercial ketone product specifications often require that aldehydic impurities be present in the ketone only at the parts per million level. Economic separation of aldehyde(s) from ketone(s) by one of the above methods to meet these specifications is often very difficult. These methods all rely on a physical or chemical operation which is not specific for the aldehyde, and which thereby affects the ketone. These methods therefore typically result in economically unacceptable losses of the desired ketone. The dramatic selectivity requirements imposed on these physical or chemical operations, to achieve an economical removal of the aldehyde from the ketone, is best illustrated by the following example.
In this example, it is assumed that the removal operation (either physical adsorption or chemical reaction) has first order kinetics in both the aldehyde and the ketone. The integrated continuity equations for a batch or plug flow reactor, and backmixed operation respectively, are written: ##EQU1## where: a=wt. % of aldehyde in the crude ketone feed, b=wt. % loss of ketone,
c=residual aldehyde concentration, ppm
k.sub.A =aldehyde first order "rate" constant, hr.sup.-1
k.sub.K =ketone first order "rate" constant, hr.sup.-1
These equations, which allow one to calculate the required k.sub.A /k.sub.K ratio for given values of a and c, and desired value of b, are illustrated in FIG. 1 ( batch or plug flow operation) and FIG. 2 (backmixed operation) for a ketone feed which contains 1 wt. % aldehyde. These figures show that, in order to achieve 50 ppm residual aldehyde in the final ketone product at less than 5 wt. % loss of the desired ketone, a kinetic selectivity ratio ,(k.sub.A /k.sub.K) in excess of 100 is needed in the case of batch or plug flow operation. A kinetic selectivity ratio k.sub.A /k.sub.K close to 5000 is desirable for the backmixed operation. These numbers suggest that the removal operation is best carried out in plug flow mode, and explain why an aldehyde specific (k.sub.A /k.sub.K =infinity) operation would be most preferred to obtain the minimum loss of ketone.
Thus, in selecting a method for aldehyde removal, it is important that the method be highly selective, if not entirely specific towards the aldehyde, to minimize ketone loss. The requirement of high selectivity in the aldehyde is especially difficult to meet when the aldehyde is of same molecular weight as the desired ketone. None of the above-described common methods for aldehyde removal appears to be adequately selective in practice to be commercially viable.
In contrast with the above prior art methods for aldehyde separation from ketones, the use of a Tischtschenko condensation of the aldehyde into a higher boiling ester product, followed by overhead distillation of the ketone to separate it from the high boiling ester product, is particularly well suited to separating aldehydic impurities from ketones, since this reaction provides substantially higher reaction selectivity. The carbonyl group of ketones does not have alpha-hydrogen atoms attached to the carbon atom; therefore, ketones cannot undergo the Tischtschenko reaction. In contrast with the above described prior art methods for aldehyde separation from ketones, the Tischtschenko condensation of aldehyde containing ketones is, therefore, anticipated to be highly selective, if not specific, toward the aldehyde.
While, in the preferred embodiments of this invention, this method may be advantageously applied to purification of ketones produced via olefin oxidation in the presence of precious metal catalysts, the selective condensation of aldehydes in the presence of ketones is more generally applied. It finds independent value in ketone production and purification via other reaction routes, such as the direct oxidation of alkanes (e.g., butane oxidation to methyl ethyl ketone and acetic acid). Also, while it is a specific intent to remove aldehydic impurities from desired ketones, the method of the present invention also applies to aldehyde/ketone streams in which the aldehyde concentration exceeds impurity-level concentrations, for example up to 10 wt. % aldehyde concentration or more.
One limitation of the present invention is that Tischtschenko condensation catalysts require substantially dry crude ketone streams to efficiently condense contained aldehydic components into heavier (and more easily separable) ester products. This limitation, however, is not overly limiting since various methods are available to create a dry, ketone-rich stream from a wet, ketone-rich stream. Among them are thermal distillation, azeotropic distillation, salting out the ketone from water by saturating the system with a salt such as sodium chloride or calcium chloride, as well as combinations of these methods. Adsorption of the water on drying agents such as molecular sieves can be used. As explained more fully below, extraction of the ketone and aldehydes with an organic solvent has been found to be an efficient "drying" method. Extraction is effective, economic, and easily integrated into a total ketone manufacturing process. Suitable extraction solvents for obtaining dry ketone streams will be those solvents having low miscibility with water and in which the ketone product is highly soluble. Such solvents include alkanes, cycloalkanes, aromatics, and their chlorinated derivatives.
One prior art reaction scheme for making ketones via partial olefin oxidation is the Wacker process. (See Chemical Engineering News, Vol. 50, July 8, 1963.) In the Wacker process, catalyzed by palladium salts, cupric chloride is utilized as the oxidizing agent in a stoichiometric manner, with the resulting cuprous chloride being subsequently reoxidized in a separate reactor (a one-stage reactor scheme). In another version of the Wacker process also using a palladium salt catalyst, oxygen is co-fed with the olefin and the aqueous catalyst solution, so that the cuprous chloride is continuously reoxidized to cupric chloride, thus using the cupric chloride in a catalytic manner (a one-stage reactor scheme). Alternatively, reduced chloride systems are described in Murtha U.S. Pat. No. 4,507,507, and in our U.S. Pat. No. 4,720,474 and Vasilevskis et al U.S. Pat. No. 4,723,041.
Regardless of which of these olefin oxidation processes is used to obtain ketones, a common characteristic of these systems is the makeup of the oxidation reaction mass, or corresponding crude ketone product mixture. Typically, this crude ketone product mixture, subsequent to the oxidation reaction, consists of an aqueous phase containing a palladium catalyst and all, or part of the ketone product. In addition, a ketone-rich organic phase containing unreacted feed components and other oxidation process components (diluents, surfactants, etc.) will form if the ketone concentration exceeds solubility limits in the aqueous phase. The crude ketone product will generally contain as impurities: water, an aldehyde of the same molecular weight as the ketone, organic acids, and other olefin oxidation products.
If the ketone is obtained via olefin oxidation in an aqueous medium, such as in the Wacker process, it is necessary to recover the desired ketone from the crude oxidation reaction mass, including the dissolved catalyst components. Further, efficient and economic recovery and purification of the ketone, product made via such aqueous phase olefin oxidation may be complicated by formation of ketone/water azeotropes, making application of simple thermal distillation techniques impractical to separate the desired ketone, the aqueous phase and other organic oxidation reaction byproducts.
Important olefin oxidation process economics factors affect the separation of the desired ketone products from the crude ketone product. Quantitative recovery of any precious metal catalyst components from the olefin oxidation reaction mass is of paramount economic concern, requiring recovery of substantially all of the precious metal catalyst components. This factor makes it economically advantageous to recover these catalyst metals in a physical state in which no additional processing is required to place them in condition for recycle to the oxidation reactor. Separate catalyst recovery and reactivation steps can introduce the risk of further catalyst losses.
When the desired ketone is sufficiently volatile, it may be separated from the crude olefin oxidation reaction mass by flashing. However, ketone recovered by this flashing technique will still be wet, containing water that flashes over along with the desired ketone. When the desired ketone is less volatile than water, the ketone must be recovered from the crude oxidation reaction mass by means other than flashing. Even if the ketone forms a second liquid phase in the crude reaction mass, there will be much ketone left in the aqueous phase. Thus, adequate recovery of ketone from the aqueous phase is a requirement of any economic process.
In designing process equipment for obtaining ketones from olefins, it is also convenient and economically favorable for the process equipment to be adaptable to a variety of ketone and/or aldehyde products and catalyst systems.
The present invention provides a method by which substantially all of the aldehydes are removed from a dry crude ketone-rich stream by use of the Tischtschenko condensation reaction. The present invention also provides a method for obtaining a dry crude ketone-rich substrate suitable for the removal of aldehydic impurities via Tischtschenko condensation. The present invention provides an integrated olefin-to-ketone process which satisfies the aforementioned process economic concerns. In one embodiment of the present invention, an extraction step, which when coupled with thermal distillations to make pure ketone product, creates a dry substrate suitable for treatment by Tischtschenko condensation. This extraction step also serves to separate the precious metal oxidation catalyst from the desired ketone products and oxidation byproducts without detrimental loss of precious metal catalyst components.
According to the present invention, aldehydes are removed from aldehyde-containing ketone mixtures by Tischtschenko condensation of the aldehyde into an ester, followed by overhead distillation of the ketone. An extraction step provides substantially dry, crude ketone to the Tischtschenko condensation step. In the extraction, the desired ketone product and oxidation byproducts are extracted to form an organic extractant phase while the precious metal catalyst remains in a second, aqueous raffinate phase suitable for recycle to the olefin and/or catalyst oxidation step without further treatment. Moreover, the extraction step advantageously creates a substantially dry ketone-rich stream, thereby eliminating the need to "break" any ketone-water azeotrope in order to obtain essentially dry pure ketone product. Subsequent distillations to recover the extraction solvent for recycle and to reject other non-aldehydic impurities from the desired ketone product will easily create the substantially dry ketone-rich stream suitable for Tischtschenko condensation of the aldehyde.
If the desired ketone product is suitably more volatile than water so that it can be economically recovered from the ketone crude by distillation, the extraction step can be advantageously applied to the flash-recovered wet crude ketone stream. This procedure, in combination with the normal distillations required for solvent and product recovery, will provide the substantially dry ketone-rich stream for subsequent treatment by Tischtschenko condensation.
Therefore, it is an object of this invention to provide a method to efficiently separate desired ketone products from their related aldehydic analogs by Tischtschenko condensation, without regard to the source of the aldehyde-containing ketone stream.
It is a further object of this invention to provide a substantially dry ketone-rich stream for contact with a Tischtschenko catalyst system to thereby condense aldehydic byproducts into higher boiling ester products, more easily separated from the desired ketone products by thermal distillation than the aldehyde substrate.
It is a still further object of this invention to provide an economically efficient process for the manufacture and recovery of high purity ketone products made by aqueous phase olefin oxidation in the presence of precious metal catalysts.
It is another object of this invention to provide a means for recovering and recycling substantially all of the precious metal oxidation catalyst in an active, usable form without deleterious accumulation of oxidation byproducts in the oxidation catalyst recycle stream.
It is an object of this invention to utilize an extraction solvent which, when recycled in small amounts to the olefin oxidation reactor, in small amounts with the precious metal catalyst recycle stream, does not adversely affect the olefin oxidation reaction kinetics.
It is another object of this invention to provide a simple overall process scheme to separate substantially dry ketone products from the olefin oxidation reaction mass while avoiding the formation of undesirable azeotropic composition of water and ketones.
It is an object of this invention to utilize an extraction solvent which enhances the separation of the desired ketone product and olefin oxidation byproducts from the other components of the olefin oxidation reaction mass.
It is yet a further object of this invention to provide a ketone separation scheme which is easily adaptable to a variety of olefin-ketone pairs and precious metal catalyst systems.
These and further objects of the present invention will become apparent to those skilled in the art with reference to the figures and detailed description which follow.